The present invention relates generally to refractory compositions that are resistant to molten metal, and more particularly to a cristobalite-free, mullite grain that is non-reactive with molten aluminum.
Conventional refractories used for the melting, alloying, containing and refining of molten aluminum and its alloys are susceptible to penetration, reaction and erosion by the molten aluminum. The aluminum, and alloying ingredients within the molten aluminum, penetrate the surface of conventional refractories and react with the refractory components. The result is the formation of compounds such as corundum (Al2O3) and spinel (MgOAl2O3) that expand and cause the refractory to crack. The cracking allows deeper penetration by the molten aluminum into the refractory causing further chemical reaction.
In addition, the foregoing compounds (i.e., the corundum and spinel) induce the formation of aluminum oxide on the surface of the refractory, as a result of the oxidation of the molten aluminum. This build-up of aluminum oxide interferes with the operation of the furnace and is therefore undesirable. Still further, the molten aluminum that penetrates the refractory can also reduce other oxides, such as silica (SiO2) and iron oxide (Fe2O3), that are present in the refractory. This results in the formation of corundum and solvation of the Si and Fe in the molten aluminum that results in embrittlement of the solid aluminum alloy. Heretofore, refractories with minimal wetting characteristics were produced using conventional refractory grains (that are normally susceptible to wetting by molten aluminum) by adding non-wetting materials as fines in the matrix of the refractory. The purpose of these fines is to retard the penetration of the molten aluminum into the refractory. For example, the addition of spinel to high alumina refractories slows the aforementioned reactions by presenting xe2x80x9cpre-reactedxe2x80x9d material to the molten aluminum. The levels of silicon and iron in such compositions are minimized to avoid reduction and dissolution of these phases from the refractory to the molten metal. However, the cost of such refractory systems is high, and such refractory systems still do not prevent the external dross attachment known as xe2x80x9ccorundum growth.xe2x80x9d
It has also been known to add boric acid and phosphates to a refractory system to minimize the porosity thereof and to slow the aforementioned reactions. In most cases, these additives have been included in unfired products because the firing process volatilizes these materials or renders them ineffective.
The foregoing additives find advantageous use primarily with refractory systems having moderate to high alumina content. Refractory grains having higher alumina content are relatively expensive, but tend to react slowly to the aforementioned reduction and penetration by the molten aluminum. Nevertheless, these high alumina materials will react with, and eventually be penetrated by, the molten aluminum. In refractory systems with lower alumina content, the silica typically, found in greater amounts in such refractory systems, is more readily reduced and dissolved in the molten aluminum. The remaining alumina of this system reacts with the alloying ingredients. Thus, in refractory systems known heretofore, the refractory grain (even high alumina grain) was susceptible to attack by molten aluminum, and efforts to reduce such attack consisted primarily of trying to minimize the porosity of the refractory system to reduce the exposure of the refractory grain to the molten aluminum.
The present invention overcomes these and other problems and provides a refractory grain that is less susceptible to wetting and attack by molten aluminum. Still further, the present invention provides a method of forming a cristobalite-free mullite grain suitable for other refractory applications.
In accordance with the present invention, there is provided a method of calcining kaolin, comprising the steps of:
a) producing a clay slurry comprised of water and kaolin clay of a specific particle size;
b) thoroughly mixing an alkaline earth metal compound selected from the group consisting of barium, strontium and a mixture thereof with said clay slurry, said compound being added in an amount wherein the barium or strontium compound content of said slurry is between about 2% by weight and about 25% by weight of the kaolin clay in the clay slurry;
c) removing sufficient water from the slurry to form a dehydrated kaolin clay-based material having a consistency that can be shaped;
d) shaping the dehydrated kaolin clay-based material; and,
e) heating said dehydrated kaolin clay-based material by continuously moving the shapes through a furnace having a zone, wherein the shapes are exposed to a temperature of at least 2,650xc2x0 F. (1,455xc2x0 C.), for at least 30 minutes.
In accordance with another aspect of the present invention, there is provided an inorganic mixture for forming a refractory grain, comprised of beneficiated kaolin clay that is essentially free of sand and impurities. The kaolin clay provides a source of alumino-silicate to the inorganic mixture. A compound additive containing at least one element selected from the group consisting of barium and strontium is added to the clay, wherein the alumino-silicate in the kaolin clay comprises about 50% to about 98% by weight of the inorganic mixture, and the at least one element in the compound additive comprises about 2% to about 25% by weight on an oxide basis of the inorganic mixture.
In accordance with another aspect of the present invention, there is provided a method of making a cristobalite-free, mullite grain having diminished reaction with molten aluminum, comprising the steps of:
a) producing a clay slurry comprised of water and kaolin clay of an average particle size;
b) thoroughly mixing a water insoluble barium or strontium containing compound with the clay slurry, the barium- or strontium-containing compound having an average particle size about equal to the average particle size of the kaolin clay within the clay slurry and being added in an amount, wherein the barium or strontium content of the clay slurry is equal to between about 3% and about 20% by weight of the kaolin clay in the clay slurry;
c) removing sufficient water from the slurry to form a dehydrated kaolin clay-containing mixture having a pelletizable consistency;
d) pelletizing the dehydrated kaolin clay-containing mixture into pellets;
e) heating the pellets by continuously moving the pellets through a furnace having a zone, wherein the pellets are exposed to a temperature of at least 2,650xc2x0 F. (1,455xc2x0 C.), and wherein the pellets are exposed to the zone for at least 30 minutes.
In accordance with another aspect of the present invention, there is provided a method of producing a cristobalite-free, mullite grain, comprising the steps of:
a) preparing a kaolin clay slurry comprised of water and kaolin clay of a predetermined average particle size;
b) thoroughly mixing a barium- or strontium-containing compound with the kaolin clay slurry, the compound being selected from the group consisting of barium carbonate, barium chloride, barium chromate, barium hydroxide, barium sulfate, barium oxide, strontium carbonate, strontium oxide, strontium sulfate, strontium hydroxide and mixtures thereof;
c) heating the kaolin clay at a sufficient temperature for a sufficient period of time to:
i) dehydrate the clay,
ii) dehydroxylate the dehydrated clay to form metakaolin,
iii) break down the metakaolin to form mullite and concomitantly form a liquid phase material comprised of silica; and,
d) cooling the mullite and the liquid phase material.
In accordance with another aspect of the present invention, there is provided a method of producing a cristobalite-free bonded mullite grain from kaolin clay, comprising the steps of:
a) thoroughly blending kaolin clay, water and a barium- or strontium-containing compound in predetermined amounts to form a pelletizable mixture;
b) forming the mixture into pellets to a size and shape suitable for calcining in a kiln to prevent dusting;
c) calcining the mixture in a kiln under time and temperature conditions sufficient to cause the kaolin clay to form mullite and silica; and,
d) further heating the mixture until the silica and the barium- or strontium-containing compound react to form a liquid phase material that wets the mullite.
In accordance with another aspect of the present invention, there is provided a refractory material comprised of mullite crystals in a cristobalite-free matrix, the mullite crystals identifiable by X-ray diffraction peaks at 2xcex8=16.5xc2x0, 26.5xc2x0, 31xc2x0, 33.3xc2x0, 35.3xc2x0 and 40.9xc2x0, and the cristobalite-free matrix being comprised of the following material:
alumina (Al2O3);
silica (SiO2); and
at least 2% by weight of an oxide selected from the group consisting of barium oxide (BaO) and strontium oxide (SrO).
It is an object of the present invention to provide a refractory grain for the production of refractory articles.
It is another object of the present invention to provide a refractory grain as described above that is resistant to attack by molten aluminum.
It is another object of the present invention to provide a refractory grain as described above that is formed from an alumino-silicate source, such as kaolin clay.
It is a still further object of the present invention to provide a method for producing the refractory grain described above.
It is a still further object of the present invention to provide a method of producing a cristobalite-free mullite grain.
These and other objects and advantages will become apparent from the following description of a preferred embodiment of the present invention taken together with the accompanying drawings.